Light amplifier including a layer for converting x-rays to visible radiation



June 11. 1968 TADAO KOHASHI 3,388,256

LIGHT AMPLIFIER INCLUDING A LAYER FOR CONVERTING XRAYS TO VISIBLERADIATION Filed. July 29, 1964 INVENTOR 13.4.3.0 KOA 8.511.;

MMM MW ATTORNEY United States Patent 3,388,256 LIGHT AMPLIFIER INCLUDINGA LAYER FOR CONVERTING X-RAYS T0 VISIBLE RADIATION Tadao Kohashi,Yokohama, Japan, assiguor to Matsushita Electric Industrial Co., Ltd,Osaka, Japan, a corporation of Japan Filed July 29, 1964, Ser. No.385,887 Claims priority, application Japan, Aug. 2, 1963, ass/41,543 3Claims. (Cl. 250213) This invention relates to light intensifiers forradiations, and particularly for those having high transmittivity, suchas X-rays, 'y-rays, etc.

The primary object of the present invention is to provide a lightintensifier of the kind specified in which radiation energy is moreefficiently utilized than ever, the sensitivity of the photoconductiveelement is never lowered by mechanical working, and impedance conditionsare also improved.

The above objects are accomplished by the inclusion of aradioluminescent layer in the device for converting the incident X-raysto visible radiation which then impinge upon aphotoconductor-electroluminescent light amplifier.

There are other objects and particularities of the present invention,which will be made obvious in the following descriptions, with referenceto the accompanying drawings, in which:

FIG. 1 is a longitudinal sectional view of a light intensifier embodyingthe present invention, with the electrical supply circuit showndiagrammatically; and

FIG. 2 is a perspective view of another embodiment of the invention,partly cut away to show the internal construction, with the electricsupply circuit shown diagrammatically.

In general, a photoconductive layer is low in transmittivity of opticalrays. As a result, a photoconductive layer effectively absorbs theenergy of optical rays irradiating the same. In other words, a photoconductive layer has a high utilization factor of optical ray energ andis effectively excited thereby. Thus a photoconductive layer has a highphotoconductive sensitivity, in general.

On the other hand, a photoconductive layer shows a certain degree ofphotoconductivity with respect to radiations, such as a-rays, 'y-rays,etc. However, the high transmittivity of such radiation causes the sameto pass through the photo conductive layer, with only a small portionbeing absorbed in the latter, resulting in low energy utilizationfactor.

In addition, the photoconduction sensitivity of the photoconductivelayer is inherently lower when excited by such radiations than whenexcited by optical rays, and consequently, the photoconductivity withrespect to such radiations is usually extremely low.

In a light intensifier for radiation of the kind specified above, inorder to obtain a high white-to-black ratio (contrast ratio) of theoutput image produced in the electroluminescent layer, it is requiredthat the electroluminescent layer has a low impedance, and thephotoconductive layer has a high dark impedance. Conventional lightintensifiers for radiations are driven by alternating current energy,and consequently, the above-mentioned impedance conditions are satisfiedby a large electric capacity of a thin electroluminescent layer, and asmall electric capacity of a thick photoconductive layer, withmechanically formed groove openings of V-shapes and a decreased spacefactor of electrodes provided on the photoconductive layer.

in general, however, the mechanical working for providing the grooveopenings in a photoconductive layer is apt to break the photoconductivematerial forming the 3,388,256 Patented June 11, 1968 layer, and such abreakage in the surface of the photoconductive layer at the side ofgroove openings extremely lowers the photoconduction sensitivity. Anymechanical working is thus undesirable for the purpose of highlysensitive operation.

Particularly with regard to the above-mentioned impedance conditions,the specific dielectric constant which determines the electric capacityof the photoconductive layer is usually higher than 10. Conventionalphotoconductive layers with groove openings formed directly thereincannot have an electric capacity other than that limited by the specificdielectric constant inherent in the material and the geometrical shapeof the photoconductive layer.

The present invention contemplates overcoming the above-mentioneddifiiculties inevitable in conventional light intensifiers forradiations.

The present invention is characterized in that a phototransparentelectrode is disposed on one side of the electroluminescent layer, andon the other side, by intermediation of an opaque layer, a currentdiffusion layer, and other necessary layers, a radioluminescent layer isdisposed for emitting optical rays by excitation with the radiation,which layer is provided with groove-shaped or pore-shaped openings, andhas a second electrode disposed on the top thereof, and in that aphotoconductive layer is arranged in electrical contact with theabovementioned second electrode and secured to the opening side of theradioluminescent layer covering the same. Voltage is applied across thephoto-transparent electrode and the second electrode, and thephotoconductive layer is excited by radiation directly, while theradiation passing through at least portions of the photoconductive layerand/or the electrodes causes the radioluminescent layer to emit opticalrays, such emitted optical rays exciting the photoconductive layer forresponding to the same.

Taking X-ray as an example of radiations, the invention will now bedescribed.

Referring to FIG. 1, the light intensifier shown comprises aphoto-transparent supporter plate 1 of transparent glass plate, forexample, and it may be of a leadcontaining glass capable of absorbingX-rays. The light intensifier further comprises a photo-transparentelectrode 2 formed by a metal oxide film, such as of tin oxide, sprayedonto the supporter plate 1, an electroluminescent layer 3 of ZnS:Cu, Alpowder for example, mixed with a bonding agent, such as epoxy resin,having a thickness of 40a or so, ZnS:Cu, Al being luminant in responseto green colors to change in electric field, and intermediate layers 41and 42, layer 41 being an insulating or semiconductive opaque layer ofblack paint or the like, having a thickness of about 5 to 10 forexample. The layer 41 serves to isolate optically the electroluminescentlayer 3 from a photoconductive layer 7, to be described later. The layer42 is a current diffusion layer for facilitating mechanical working ofopen portions 52 of a radioluminescent layer 50 to be described laterand also for causing diffusion of the photo-current to prevent stripingof output image L. This layer is formed by powder of non-linearresistance material, such as CdszCl for example, mixed with bondingmaterial, such as epoxy resin for example, and has a thickness of aboutto 300 for example.

The radioluminescent layer 59 is rendered luminant by excitation ofradiation, and is provided with open portions 52 in the form of parallelV-shaped grooves. The tops of the grooves are provided with electrodes 6formed by metal vaporizing, adhesive silver paint, or the like. Thephotoconductive layer 7 is secured to the radioluminesceut layer 50 atits side face 51 to cover the same and, at the same time, is inelectrical contact with the electrodes 6.

In the embodiment shown, the photoconductive layer 7 may be formed byvacuum vaporization, or by spraying a mixture of photoconductive powderand bending material, such as epoxy resin, diluted with a solvent, suchas diacetone alcohol, to form a thin layer.

It is now assumed that the photoconductive layer 7 is formed with aphotoconductive material of the CdS series activated by Cu, Cl.Photoconductive materials of the CdS series have photoconductivity evenfor X-rays, and have a high photoconductive sensitivity for optical rayexcitation, its spectral distribution of photoconduction being in therange of 500 to 900 m The radioluminescent layer 50 is required to emitoptical rays by the X-ray (radiation) exciting, and also to excite thephotoconductive layer 7 to respond by the emitted optical rays, thusvarying the impedance. Consequently, the spectral energy distributioncharacteristics of the optical rays should be in overlapping relation atleast partly with the spectral photoconductivity distributioncharacteristics. Thus, if the spectral photoconductivity distributioncharacteristics are in the above-mentioned range of 500 to 900 m thespectral energy distribution characteristics of optical rays of thelayer 59 should at least partly be in 500 to 900 m For the mostdesirable case, the spectral energy distribution characteristics shouldexist in the range or" the spectral photoconductivity distributioncharacteristics, and the maximum integral value should be obtained whenthe product of the respective response value (distributed value) forevery wave length has been integrated against the desired wave lengths.

For photoconductive materials of the CdSzCuCl series, X-ray luminescentphosphor, (Cd, Zn)S (solid solution of the CdS and ZnS) activated withAg satisfies the abovementioned condition. For example, when the layer 7is formed by spraying powdered CdSzCuCl, which is highly sensitive toorange light, mixed with a bonding agent, such as epoxy resin, etc., toform a suspension, the layer 50 may be formed by a powder of X-rayluminescent phosphor (Cd, Zn)S:Ag which is orange luminant, mixed withan adhesive agent, such as epoxy resin, polystyrol resin, etc. Thethickness of layer 59 is selected to be large enough to have anappropriately high impedance in comparison to that of the layer 3, tolower electroluminescent output in a dark state for improvingwhiteto-black ratio (Contrast ratio) of the output image L.

For example, the layer 5t) may be of about 300 to 400,11. thickness, andthe conductive adhesive agent is applied over one face of layer 50 to athickness of about to 40 Then, V-shaped grooves are cut therein at pitchspacings of about 600 to provide openings 52. With such a construction,the space factor of electrodes 6 is lower than 1, and therefore, thelayer 50 shOWS further high impedance by virtue of openings 52. Afterthe radioluminescent layer 50 and electrodes 6 have thus been formed,the photoconductive layer 7 is formed thereon by spraying. Theelectrodes 6 are all connected to a conductor 8 in parallel relation,while the transparent electrode 2 is connected to a conductor 10. Thus,the electrodes 6 and 2 are connected across an alternating currentsupply source 9.

When an X-ray image X is projected onto the intensifier as shown byarrows, the impedance (resistance) of the photoconductive layer 7decreases by the X-ray excitation. X-rays, being of high transmittivity,pass through the layer 7, and through the opening sides 51 of theradioluminescent layer 59 and electrodes 6 to excite the layer 5% whichin turn generates orange light L The light L excites the photoconductivelayer 7 covering the opening sides 51 to respond optically. Since thelayer 7 is of low transmittivity with respect to light, and in addition,the layer 59 shows a behavior as one kind of light integrant, theoptical rays generated in the layer 50 are wholly absorbed by the layer7 through the opening sides 51, except those which are absorbed in thelayer 56 itself, and the optical excitation is accomplished effectively.

Consequently, the photoconductive layer 7 is excited by X-rays directly,and also excited by the optical rays converted in the layer 50 frompenetrating X-rays that would otherwise be lost, and operates with ahigher sensitivity than ever. The photocurrent flows from electrodes 6along opening sides 51, and diffuses through the layer 42 to render thelayer 3 luminant. The X-ray image X has thus been converted andintensified to a visible image L on the layer 3.

According to the invention, mechanical Working for providing openings isdone only in the radioluminescent layer, but not in the photoconductivelayer which can be formed by vaporizing or spraying, and consequently,the photoconductive sensitivity is never damaged. In addition, thephotoconductive layer and the radioluminescent layer are of differentmaterials, and therefore, by appropriate construction of theradioluminescent layer, its dark impedance (impedance in a dark state)can be made high, in comparison to direct working on the photoconductivelayer for provision of groove openings therein, even if theirgeometrical shapes are the same.

For example, when the photoconductive layer is formed by aphotoconductive powder of the CdS series mixed with an adhesive agent,its specific dielectric constant can be made low to a certain extent byuse of an adhesive agent of low specific dielectric constant E, such as,for example, polystyrol of E:2.5. However, unless photoconductiveparticles have good mutual contact, high photoconductive sensitivitycannot be obtained. Usually, the mixture proportion by volume ofphotoconductive powder should be about but the specific dielectricconstant of photoconductive particles is higher than 10, andconsequently, the specific dielectric constant of the photo conductivelayer is naturally higher than 10.

According to the present invention, however, the radioluminescent layer50 serves as an impedance. Radioluminescent particles are commingled inthe layer 50, but in this case, the mixture proportion is never limitedfrom the requirement of mutural contact of the particles as in the caseof photoconductive layer. If the radioluminescent particles, such asCdZnStAg, have a specific dielectric constant of 10 or so, and anadhesive agent, such as polystyrol, of low specific dielectric constantof 2.5 or so, is used, there being no limit on the mixture proportion byvolume, the specific dielectric constant can be made lower than 10, andas low as 2.5 or so, by decreasing the proportion by volume offluorescent particles.

In the foregoing description, the dark impedance is considered from theside of electric capacity, but below is considered from the side of darkresistance.

Radioluminescent particles, such as (Cd.Zn)S:Ag, are of an insulatingnature, and have an extremely high specific resistance. On the otherhand, photoconductive materials, such as CdS:CuCl, have a darkresistance lower than the former. -It is, therefore, clear that thepresent invention is superior to the well-known art, with respect to thespecific resistance also.

'In the embodiment shown in FIG. 1, the open portions 52 are in the formof equal-spaced parallel grooves, but may be of other form, such asconcentric circles or spirals, and they are not necessarily V-shaped,but may be circular are or rectangular in cross-Section.

Further in FIG. 1, the photoconductive layer 7 is secured to and coversthe V-shaped opening sides 51, but X-rays or other radiations of hightransmittivity can pass through the layer 7 even when it issubstantially thick, and the photoconductive layer 7 is allowed to fillup the openings 52. Although the photoconductive layer 7 covers theelectrodes 6 in FIG. 1, it is not required, but the only requirement isthat the layer 7 is in electrical connection with electrodes 6.Consequently, electrodes 6 may be exposed beyond the layer 7.

'FIG. 2 shows another embodiment of the invention, in which the openportions are of pore shape. In this figure, the same reference numeralsare used for indicating parts corresponding to those in FIG. I.

Referring to FIG. 2, the open portions are regularly distributed conicalpores, with their tip portions extending into the current diffusinglayer 42. The photoconductive layer 7 lills up the conical-poreopenings, and covers the opening sides 51, and further covers and issecured to the electrode 6 of net form.

Needless to sa the photoconductive layer 7 may not fill up the openportions, but merely cover the opening sides 51, in electrical contactwith the electrode 6. The shape of the pores need not be conical, butmay be cylindrical, square-columnar, semi-spherical, or the like.

In either of the above-described two embodiments, the groove or conicalopenings extend into the current diffusing layer 42, and this isdesirable for effectively utilizing the impedance change in thephotoconductive layer 7, and also for preventing striping or spotting ofthe output image by effective diffusion of the photocurrent, but this isnot limitative. For example, the groove openings may extend to midportions of the radioluminescent layer 50, or to its face in contactwith the layer 42. Further, the current diffusing layer 42 may beomitted, if required.

Further in the embodiments shown, the electrodes 6 are X-raytransmittive (radiation transmittive), and the opening sides 51 forminclined surfaces, and therefore, the radioluminescent layer 50 isexcited to become luminant by both of the X-rays (radiations) that passthrough the portions of electrodes 6 and photoconductive layer 7,obtaining the desirable results, but the present invention is neverlimited to such a case only. Thus, the objects of the present inventioncan be accomplished principally by the radioluminescent layer 50 beingexcited to emit optical rays by radiation that passes through either theportion of electrodes 6 or the portion of the photoconductive layer 7.

From the foregoing, it is seen that the essential condition of thepresent invention is to have the radioluminescent layer excited to emitoptical rays by the radiation that has passed through at least eitherportion of the photoconductive layer 7 or electrodes 6. It is naturalthat substances of the electrodes and photoconductive layer as well asshape of the open portions should be selected to satisfy the abovecondition.

The photoconductive layer may be a sintered layer, when glass enamel orother adhesive agents durable to high temperature are employed in theradioluminescent layer. Further, in case that the electroluminescentlayer is rendered lnminant by a direct current, such as a vaporizedelectroluminescent layer, it may be connected to a DC. supply source foreffecting direct current operation.

For use for -rays and other radiations than X-rays, appropriatematerials should be employed, needless to say. The radioluminescentlayer may be formed without employing an adhesive agent, but bysintering, etc.

What is claimed is:

.1. A light intensifier for radiations comprising an electroluminescentlayer, a photo-transparent first electrode disposed on one side of saidelectroluminescent layer, a radioluminescent layer mounted on the otherside of said electroluminescent layer, a plurality of recessions in saidradioluminescent layer on the far side thereof from saidelectroluminescent layer, at least one second electrode disposed on thetop portion of the recessed radioluminescent layer, a photoconductivelayer covering the recessed surface of said radioluminescent layer aswell as its recessed portions, said photoconductive layer beingelectrically connected with said second electrode, and an electricvoltage source applied across said phototransparent first electrode andsaid second electrode.

2. The light intensifier according to claim 1, in which said recessionsof radioluminescent layer are of groove shape.

3. The light intensifier according to claim 1, in which said recessionsof radioluminescent layer are of pore shape.

References Cited UNITED STATES PATENTS 2,835,822 5/1958 William 250-2132,975,294 3/1961 Kazan et al. 250213 2,999,941 9/1961 Klasens et al250-213 3,064,133 11/1962 Murr et al. 250-2l3 3,210,551 9/ l965 Vaughnet al. 250-213 RALPH G. NILSON, Primary Examiner.

M. A. ABRAMSON, Assistant Examiner.

1. A LIGHT INTENSIFIER FOR RADIATIONS COMPRISING AN ELECTROLUMINESCENTLAYER, A PHOTO-TRANSPARENT FIRST ELECTRODE DISPOSED ON ONE SIDE OF SAIDELECTROLUMINESCENT LAYER, A RADIOLUMINESCENT LAYER MOUNTED ON THE OTHERSIDE OF SAID ELECTROLUMINESCENT LAYER, A PLURALITY OF RECESSIONS IN SAIDRADIOLUMINESCENT LAYER ON THE FAR SIDE THEREOF FROM SAIDELECTROLUMINESCENT LAYER, AT LEAST ONE SECOND ELECTRODE DISPOSED ON THETOP PORTION OF THE RECESSED RADIOLUMINESCENT LAYER, A PHOTOCONDUCTIVELAYER COVERING THE RECESSED SURFACE OF SAID RADIOLUMINESCENT LAYER ASWELL AS ITS RECESSED PORTIONS, SAID PHOTOCONDUCTIVE LAYER BEINGELECTRICALLY CONNECTED WITH SAID SECOND ELECTRODE, AND AN ELECTRICVOLTAGE SOURCE APPLIED ACROSS SAID PHOTOTRANSPARENT FIRST ELECTRODE ANDSAID SECOND ELECTRODE.